Winsford Rock Salt Mine, located west of the River Weaver at Meadowbank, is Britain’s oldest working mine and the last salt mine on the mainland. Even before the Romans arrived, salt had been produced in the area by boiling the water from the natural brine springs.

The solid veins of underground rock salt were discovered here in the 17th century, when prospectors began searching for coal to fuel their saltpans. The salt mined here is now used for gritting and de-icing roads. Some of the space left by salt extraction houses secure storage facilities.

The Triassic rock salt, or halite, at Winsford is arranged in seams some 25m thick, sometimes mixed with bands of Keuper Marl. Water has penetrated the upper seams, giving rise to the brine springs,but the deepest beds lie beneath a layer of impervious marl. The two workable seams are 130-220m below the surface, with the purest salt in the deepest part of the seam.

Formerly known as Meadow Bank Mine, Winsford Rock Salt Mine officially opened in 1844, when Shafts 1 and 2 were sunk. Each shaft was 1.2m square, and lined with timber and puddle clay. Both shafts were originally 64m deep but were later extended to 152m deep to reach better quality salt.

Men, materials and salt were all lowered and raised through the shafts in buckets. Black powder was used to blast the rock face, then the salt was dug with hand picks and shovels, and transported back to the shafts in wooden barrels. The only light was from tallow candles. Despite the hardships, the miners managed to extract 90% of the rock, leaving the remaining 10% as pillars to support the overburden.

However, by the late 19th century the salt industry was suffering from over-supply — the formation of the Salt Union in 1888 having failed to solve the problem. The mine closed in 1892, after yielded a million tonnes of rock salt. It reopened in 1928, after other local salt mines had flooded. Electricity was introduced in the 1930s, and Shaft 3 was sunk in 1941. Shafts 1 and 2 were sealed and filled in the 1970s.

Shaft 3 is circular in cross-section with a diameter of 3m and a depth to the sump of 168m, although the decking level is at 152m. This shaft is now used for access. The original brick lining has been replaced with concrete. Its balanced pair of double-deck cage lifts was upgraded to a fixed self-service lift in the 1970s, which takes a minute to descend.

Shaft 4, 1.6km east of Shaft 3, was constructed in 1963 and is used for both ventilation and materials access. Also circular, it has a concrete lining with an internal diameter of 4.9m, a depth of 189m and a decking level of 183m. The double compartment lift takes five minutes to descend, carrying people in the upper section and cargo in the lower section.

The cargo compartment is 7m high, 2.4m wide and 4m deep and can take loads of up to 15 tonnes. All machinery, including the huge excavators, is brought into the mine this way. Items have to be dismantled into lift-sized pieces and then reassembled in the underground workshops — this means that once in the mine, most machines never leave!

Shaft 5, some 100m north of Shaft 3, was completed in 1973 and is used for raising the rock salt to the surface. Similar in design to Shaft 4, it is 4.9m in diameter but only 164.5m deep with a skip loading station at 155m. Two cage-over-skip units operate in balance, each with a capacity of 9 tonnes.

Fresh air is circulated from Shaft 4 around the mine to disperse fumes from the machines and from blasting operations. Methane gas is not an issue here, unlike in coal or metal mines, and the rock naturally regulates the air temperature to a constant 14 degrees centigrade.

In 1958, the Mining Department of Newcastle University set up a programme to monitor creep (rock salt moves slowly with time) and the strata deformation that still continues. This information led to the current extraction rate of 68-75%, with typical voids 20m wide and 8-9m high and pillars 20m square.

Since the mine reopened, the production of rock salt has been an eight-stage process — undercutting, drilling, blasting, loading, scaling, crushing, elevation to the surface and dispatch/stockpiling.

Winsford uses the largest wheeled loader of any mine in the UK, the Komatsu WA800, which carries up to 18 tonnes in its bucket. In January 2002, £2m was spent on an American-made 132 tonne Joy Continuous Miner, a huge remote-controlled ‘walking’ cutting machine. It removes material neatly from the rock face and passes it onto a conveyor belt linked to the crushing plant, reducing production to just four stages. By 2012 it may need replacing.

The crushed salt is separated into two grades: 0-6mm and 0-10mm diameter. It is treated with an anti-caking additive and then it’s ready to use.

Today the mine covers an underground area of 5km east to west and 3km north to south, with some 23 million cubic metres of void space connected by 222km of tunnels. It is estimated that 50 million tonnes of rock salt has been extracted in the past 50 years.

The stable temperature and humidity in the mine, together with its size and security, make it an ideal place for storage. In 1998, the company DeepStore began storing boxes of information in some of the ‘rooms’ within the mine. There are now 2 million boxes (although there is space for many more) containing all kinds of things — documents, police evidence, paintings, scale models, data and some of the National Archives from Kew.

After 2005, when the mine passed a 50,000-year environmental assessment, 2 million cubic metres were set aside for waste storage. The toxic ash residue from household incineration is sealed into 1.2 or 1.5 tonne white polypropylene bags, which are packed like bricks into the voids between pillars.

Winsford Rock Salt Mine is not open to the public, owing to the Health & Safety Regulations governing working mines. The locations of the various secure storage areas within the mine are top secret.